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A New, Non-Destructive Method for Sampling Burrowing Crayfish
Jonathan D. Hopper and Alexander D. Huryn

Southeastern Naturalist, Volume 11, Issue 1 (2012): 35–48

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2012 SOUTHEASTERN NATURALIST 11(1):43–48 A New, Non-Destructive Method for Sampling Burrowing Crayfish Jonathan D. Hopper1,* and Alexander D. Huryn1 Abstract - Burrowing crayfish are typically sampled by excavation, which results in habitat destruction. Several less destructive trapping methods have been devised, though their results have been inconsistent in our experience. To conduct studies of growth and longevity, a more efficient method was needed to sample burrowing crayfish while leaving their burrows intact. We describe a new trap design based on a reversed pitfall trap. This trap was tested using a population of burrowing crayfish, Cambarus Lacunicambarus diogenes (Devil Crayfish), in Alabama. The weekly trapping success rate (n = 18 traps) over a 4-week period ranged from 6% to 33%, with an average success rate of 17%. We feel that the implementation of our design will enhance studies requiring nondestructive sampling. Introduction Crayfish classified as primary burrowers contribute 15% to total crayfish species richness, while representing >30% of those listed as critically imperiled (Eversole and Welch 2010, Hobbs 1981, Hobbs and Lodge 2010, Welch and Eversole 2006b). Despite their high level of imperilment compared with other crayfish, relatively little is known about their biology. This lack of knowledge has been partly caused by the difficulty involved in capturing burrowing species (Duffy and Thiel 2007), as indicated by the relatively large disparity between the number of ecological studies devoted to stream- and lake-dwelling species versus burrowers. At present, the most effective method for capturing burrowing crayfish is excavation (Ridge et al 2008). There are obvious drawbacks to this method; besides being labor and time intensive, roots or stones in the soil matrix surrounding the burrow often prohibit excavation. Moreover, this method destroys the burrow, a habitat that is sometimes used by juveniles and other organisms (Duffy and Thiel 2007). Due to the drawbacks of excavation, a number of burrowing crayfish traps have been designed (Hobbs 1972, Norrocky 1984, Ridge et al 2008, Welch and Eversole 2006a). The Norrocky burrowing crayfish trap is based on a hinged one-way trap door in a tube that is inserted into the burrow’s opening (Norrocky 1984). This trap captures the crayfish within the tube as it makes its way past the trap door. Norrocky (1984) reported a trap success rate of 13%, but provided no further details of how this was estimated. The burrowing crayfish net (Welch and Eversole 2006a) is based on the insertion of avian mist netting into the burrow mouth, which entangles and captures crayfish as they attempt to exit. This method is ideal when trapping 1University of Alabama, Department of Biological Sciences, Box 870206, Tuscaloosa, AL 35487. *Corresponding author - jdhopper@crimson.ua.edu. 44 Southeastern Naturalist Vol. 11, No. 1 species with large numbers of spines and tubercles, although it shows less promise when trapping smoother-bodied species (Ridge et al. 2008). Welch and Eversole (2006a) compared the burrowing crayfish net method with the Norrocky trap (n = 25 of each trap type, deployed over 7–12 days for 4 sampling periods from February to June 2001), resulting in an average weekly capture success rate of 4% for the Norrocky trap and 20% for the burrowing crayfi sh net. Capture success was estimated as the mean of the percent of successful traps (# crayfish captured/traps deployed) for the four deployments. Ridge et al. (2008) compared the two methods based on traps deployed at 268 pairs of burrows and reported capture success rates (# crayfish captured/traps deployed) of 4.5% for the crayfish net and 5.2% for the Norrocky trap. Based on these studies, the Norrocky trap appears to have a somewhat consistently low success rate. The capture success rates of the crayfish net are variable, but can be relatively high (Welch and Eversole 2006a). In our studies of crayfish in the Sipsey River drainage (Tuscaloosa County, AL), we attempted to use the crayfish net and the Norrocky trap to capture crayfish. In our experience based on sampling Cambarus (Lacunicambarus) diogenes Girard (Devil Crayfish), however, the excavating activities of crayfish produced significant problems for these trap designs. The net traps were routinely encased in clay and pushed from the entrances of the burrows and the tubes of the Norrocky trap became filled with clay rendering the trap door non-functional. To circumvent problems with these previously devised methods, we designed a new trap. Methods Study site Our study was conducted in an open canopy roadside ditch beside the Taylor Creek headwater stream. This stream is a tributary to the Sipsey River in western Alabama (33.0964°N 87.8325°W). The site is a well-drained portion of the Sipsey River flood plain and rarely floods for prolonged periods. The study area is a roadside drainage ditch approximately 175 m2 in area containing an average of 0.65 burrows/m2 inhabited by C. Diogenes. The diameters of the burrow openings ranged 18–49 mm (mean = 29 mm, n = 28). Trap design and construction Our design is based on a reversed pitfall trap. This trap is constructed of a covered bucket with a hole in its bottom. The bucket is placed over a crayfish burrow, and a modified funnel is inserted into the burrow’s opening. Crayfish emerging from the burrow fall from the top of the funnel to become trapped within the bucket (Fig. 1). Traps were constructed from black plastic 2-gallon (7.75-L) buckets with 20-cm diameter bottoms. The upper part of the bucket was removed, resulting in a cylinder 10-cm in height. A 7-cm diameter hole was cut into the center 2012 J.D. Hopper and A.D. Huryn 45 of the bottom of the bucket and a funnel was glued into the hole using Liquid Nails®. Roughly 1-cm of the narrow part of the funnel protruded from the bottom of the bucket. The funnel was modified so that the top was 14-cm in diameter and the bottom was 6-cm in diameter. These dimensions were appropriate given the size of C. diogenes burrows and provide enough room for the crayfish to move over the top of the funnel when the trap is closed with a tight-fitting lid. Covering the trap minimized evaporation of water and entry of predators such as birds and raccoons. Trap deployment The burrow’s chimney, if present, was removed and the trap was placed over the burrow opening and pressed firmly into the sediment. Traps were then partially filled with water to keep captured crayfish from desiccating. Water was also poured down the burrow itself. In our experience, this enhances crayfish activity within its burrow and may increase the probability of capture (J.D. Hopper, pers. observ.). Figure 1. Upper left: A pair of crayfish traps deployed on burrows of Cambarus diogenes on the Sipsey River floodplain in Tuscaloosa County, AL. Upper right: Open crayfish traps showing evidence of crayfish activity (left trap), with the actual capture of the burrow’s resident (right trap). Lower left: Close up of crayfish trap showing evidence of crayfish activity without actual capture of the burrow’s resident. Lower right: Close-up of trap containing captured specimen of C. diogenes. 46 Southeastern Naturalist Vol. 11, No. 1 Recently, it has been suggested that the burrow chimney is of more importance to the crayfish than simply being a deposition point of sediment (B. Helms, Department of Biological Sciences, Auburn University, Auburn, AL, pers. comm.). To deploy the trap, however, the chimney must be removed to allow the bottom of the trap to be flush with the soil. One of the aims of using burrow traps is to limit stress on the crayfish in order to conduct long-term studies. Consequently, chimneys were gently lifted away and placed to the side of the trap during deployment. Once sampling was completed, chimneys were replaced. In the event the chimney serves a purpose, its replacement post-sampling should decrease sampling stress on the population during longterm monitoring efforts. In April 2010, we used our new trap design to sample a population of burrowing Cambarus diogenes within the Sipsey River drainage in Tuscaloosa County, AL. Eighteen traps were deployed and checked weekly for four weeks. Traps were graded as “empty” if there was no evidence of crayfish activity, “empty with evidence of activity” if chimney construction within the funnel was evident, or “successful” if a crayfish was captured. When traps were found empty after being deployed for 1 week, they were moved to a new burrow. Traps containing crayfi sh were also moved to new burrows to avoid recapturing the same individual over and over. Traps containing sign of crayfish activity were cleaned of excess sediment and re-deployed on the same burrow. Table 1. Capture success rate for new trap design. Traps were deployed for four weeks during April 2010. # captured = number of crayfish captured using 18 traps. # activity = number of traps with evidence of activity (e.g., crayfish captured or construction of a burrow chimney within the trap’s funnel). # no activity = number of traps showing no evidence of crayfish activity. Week # captures # evidence # no activity 1 6 8 4 2 3 9 6 3 2 11 5 4 1 11 6 Table 2. Trapping efficiency by trap form. A) Ridge et al. (2008) trapping efficiency over the course of their study. B) Welch and Eversole (2006a) weekly average trapping efficiency computed from their data. C) Norrocky (1984) success rate, no data or date range given. D) Weekly average trapping efficiency for this study. Weekly average trapping efficiency Trap form A B C D Norrocky 5.20% 4.04% 13.00% - Mist net 4.50% 20.08% - - New design - - - 16.60% 2012 J.D. Hopper and A.D. Huryn 47 Results We captured 12 specimens of Cambarus diogenes during the month of April 2010 (Table 1). Traps were checked 4 days after the initial deployment. Fifteen of these traps displayed evidence of crayfish activity, yet only one individual was captured. Seven days after deployment, traps were checked again (following rain), and an additional 5 crayfish were captured. The rain also apparently prompted crayfish to increase their digging activities resulting in the partial filling of some traps with sediment, though there was no evidence of crayfish being caught and subsequently escaping. The number of crayfish captured per week ranged from 1 to 6 individuals. Our weekly capture success thus ranged from 6% to 33%. The average weekly trap success rate during the entire 4-week study was 17%. Interestingly, the highest number of captures took place during the first week of sampling, after which the number of crayfish captured steadily declined, possibly due to the exhaustion of active burrows. During the first week of deployment, one trap was found with two crayfish captured from a single burrow. During the second week of deployment, one dead crayfish was found in a trap. All water in this trap had evaporated, and the crayfish was apparently dehydrated. This incident was the only occurrence of mortality. Discussion In our experience, the Norrocky trap and the burrowing crayfish net were not successful in capturing C. diogenes from their burrows. We attribute this failure to the large amounts of clay excavated by crayfish during burrow construction and maintenance. Our new trap design provides room for the accumulation of excavated sediment while remaining functional. In addition, it leaves the burrow intact, requires only temporary removal of the chimney, and provides a waterfi lled and predator-free refuge for captured specimens. Finally, it is inexpensive and relatively easy to construct. Given the capture success rate of our new trap, compared with other published designs (Table 2), it provides an effective alternative method for sampling burrowing crayfish. Literature Cited Duffy, J.E., and M. Thiel. 2007. Evolutionary Ecology of Social and Sexual Systems: Crustaceans as a Model Organism. Oxford University Press, Oxford, UK. Eversole, A.G., and S.M. Welch. 2010. Conservation of imperiled crayfish: Distocambarus (Fitzcambarus) youngineri Hobbs and Carlson 1985 (Decapoda: Cambaridae). Journal of Crustacean Biology 30:151–155. Hobbs, H.H., Jr. 1972. Crayfishes (Astacidae) of North and Middle America. Biota of Freshwater Ecosystems. Identification Manual No. 9. US Environmental Protection Agency, Washington, DC. 173 pp. Hobbs, H.H., Jr. 1981. The Crayfishes of Georgia. Smithsonian Contributions to Zoology 318. 48 Southeastern Naturalist Vol. 11, No. 1 Hobbs, H.H., III, and D.M. Lodge. 2010. Chapter 22. Decapoda. Pp. 901–967, In J.H. Thorp and A.P. Covich (Eds.). Ecology and Classification of North American Freshwater Invertebrates, Third Edition. Academic Press, Elsevier, London, UK. Norrocky, M.J. 1984. Burrowing crayfish trap. Ohio Journal of Science 84:65–66. Ridge, J., T.P. Simon, D. Karns, and J. Robb. 2008 Comparison of Three Burrowing Crayfish Capture Methods Based on Relationships with Species Morphology, Seasonality, and Habitat Quality. Journal of Crustacean Biology 28:466–472. Welch, S.M., and A.G. Eversole. 2006a. Comparison of two burrowing crayfish trapping methods. Southeastern Naturalist 5:127–30. Welch, S.M., and A.G. Eversole. 2006b. The occurrence of primary burrowing crayfish in terrestrial habitats. Biological Conservation 130(3):458–464.